(1) Field of the Invention
The present invention relates to a magnetic reproducing apparatus for reproducing a digital signal from a magnetic recording medium in which digital signals are modulated and recorded as modulated digital signals, and more detailedly relates to a reproduced waveform equalization circuit which processes modulated digital signals detected by a reproducing head.
(2) Description of the Prior Art
In a digital magnetic recording device such as a digital compact cassette (to be referred to hereinafter as DCC) device etc., digital signals are modulated to produce modulated signals, which in turn are recorded on a magnetic recording medium. When the modulated data is detected from modulated signals upon reproducing, a correction called reproduced waveform equalization is performed so that the transfer function for the recording/reproducing process may have Nyquist characteristics in order to inhibit interference between codes. This correction can be done by using either an analog filter or a digital filter. FIG. 1 shows a prior art configuration in which the reproduced waveform equalization is performed by a digital filter.
In the figure, a head amplifier 1 amplifies the modulated signal from a reproducing head to a preset level. A low-pass filter (to be referred to hereinafter as an LPF) 3 is used to attenuate high-frequency components beyond required ranges in order to remove aliasing noise generating in sampling at an A/D converter 64a. The output from the A/D converter 64a is introduced to a finite impulse response digital filter (to be referred to hereinafter as an FIR filter) 66. This FIR filter 66 is also called as a transversal filter and performs equalization in digital mode. The output from the FIR filter 66 is introduced to a data detector in which predetermined modulated data is produced.
FIG. 2 shows an improved configuration of the prior art configuration shown in FIG. 1. In this configuration, a high-pass filter (to be referred to hereinafter as an HPF) 2 attenuating low-frequency components of the reproduced modulated signal is interposed between the head amplifier 1 and the LPF 3. Further, an infinite impulse response digital filter (to be referred to as an IIR) 65 compensating the attenuation of the low-frequency components due to the HPF 2 is interposed between the A/D converter 64b and the FIR filter 66.
Advantages in the above configuration will be described hereinbelow.
In general, a modulated signal is composed of several kinds of frequencies, although the number of frequencies depends on its modulating mode. For example, in a case of the 8/10 modulating mode which has been used for the recent DCC devices, the maximum cyclic frequency is 48 kHz (to be referred to hereinbelow as fmax) while the minimum cyclic frequency is 9.6 kHz (to be referred to hereinbelow as fmin). As to the reproduced modulated signal, although the levels of the reproduced signal at the frequencies of fmax and fmin depend upon electromagnetic characteristics such as frequency characteristics of a reproducing head used, the reproduced signal level at fmin is typically greater than that at fmax. In the case of DCCs, this level difference is greater than some 20 dB.
In this case, as regards the S/N ratio of the signal A/D converted by the A/D converter, the level of the signal component S in the S/N ratio will be determined by the reproduced signal level at fmax while the level of the noise component N in the S/N ratio will be determined by noises due to the quantization by the A/D converter. This quantization noise is determined by the dynamic range which can be determined by the number of bits for quantization in the A/D converter and the aforementioned reproduced signal level at fmin.
Based on the above description, a comparison will be made between the prior art configurations shown FIG. 1 and FIG. 2 in view of the number of bits in the A/D converters 64a and 64b.
For instance, suppose that a reproduced signal having levels at fmax and at fmin as shown in FIG. 3 is to be obtained as the output from the head amplifier 1 shown in FIG. 1. A cut-off frequency of the LPF 3 is set up so high as compared to fmax as not to affect both reproduced signal level at fmax and fmin. Since the reproduced signal levels at fmax and fmin will not change in their level after being processed through the LPF 3, the reproduced signal in the A/D converter 64a in the configuration of FIG. l, has a dynamic range of `m` dB or the difference between the quantization noise level and the reproduced signal level at fmin. Therefore, the converter 64a has to have a corresponding number of bits for quantization which is able to represent the dynamic range of `m` dB.
As a general property of A/D converters, when a dynamic range is represented by `m` (dB) and a number of bits for quantization is represented by `k`, there holds a relation m=6.times.k. Accordingly, the aforementioned A/D converter 64a requires m/6 bits.
On the other hand, in the configuration shown in FIG. 2, the reproduced signal level at fmin is reduced after passing through the HPF 2 by the inhibiting effect of low-frequency components due to the HPF2. At that time, if the attenuating amount of the reproduced signal level is `h` dB, the dynamic range required for the A/D converter 64b is (m-h)dB and therefore the required number of the bits for quantization for the A/D converter 64b is (m-h)/6.
Now, suppose the DCC device operates in the 8/10 modulating mode and the HPF 2 presents an attenuation characteristic of -6 dB/oct., the aforementioned value `h` dB can be calculated as about 14 dB. As a result, it is possible to decrease the number of bits in the A/D converter 64b by 2.3 bits as compared to that in the A/D converter 64a.
In the prior art shown in FIG. 2, the provision of the HPF2 narrows the dynamic range to be required for the A/D converter, whereby the number of the bits for quantization required for the A/D converter 64b are decreased. Nevertheless, as the output from the A/D converter 64b is directly introduced to the IIR filter 65 for compensating the reduction in the low-frequency component, the low-frequency range is emphasized in this IIR filter 65 so that the reproduced signal level at fmin is increased by `h` dB. Accordingly, the output increased in its dynamic range by the IIR filter 65 is input to the FIR filter 66, so that the number of input bits to FIR filter 66 becomes large. This disadvantageously requires a large-scaled circuit configuration for the FIR filter 66.